22 Issue 216 July 2008 CIRCUIT CELLAR® www.circuitcellar.com IEIE

can spend time in New Mexico and see if we want to makethe move permanent. The property has an existing well,which took a big risk out of the decision to purchase. Wecould theoretically drill at about $20 per foot and hit nothing.So, having the well in place removed a big gamble in my mind.

My wife is a trusting soul or I’m very convincing. I assuredher that we could build a house and I could get the water andsolar working like you would never know we were off the grid.No fuss, no muss, no problem. Well, a lot is really riding onthis project. Let’s call it domestic tranquility. And I also don’twant to give up my day job designing embedded systems.

How does one go about planning for an off-the-grid solarsystem? After reading through some books and magazineson the subject, I soon realized that this was just anotherproject, like many others I’ve completed in my career. Thebooks and magazines didn’t have the engineering detailsthat I required. As we visited the property and talked tobuilders, we met our neighbors, who were also off the grid.

My wife and I are getting to the age at which we arelooking for a place to retire. We live in Connecticut andhave heard lots of stories about friends who sold theirhomes and left the state to retire only to regret the deci-sion. So, our plan has been to find an area that we wouldconsider for retirement and try it out by living there on apart-time basis. Well, to make a long story short, we endedup near Silver City, New Mexico, with a nice plot of landsuitable for a retirement home (see Photo 1).

Located where the Rocky Mountains come to their southernend, the area is beautiful with four moderate seasons. We pur-chased the land at a good price. Our property is only 2 milesfrom the Gila National Forest. The New Mexico portion ofthe forest is about the size of the entire state of Connecti-cut. To say we are out in the middle of nowhere is anunderstatement. You can hear the crows breathe as they flyby. It is a perfect place for an off-gird solar home.

The plan is to build a small workshop/apartment so we

Living And Working Off The GridPart 1: Planning And Design

INTELLIGENT ENERGY SOLUTIONS

LESSONS FROM THE TRENCHES

by George Martin

George is going solar by installing an off-the-grid photovoltaic (PV) power-generat-ing system at his workshop/apartment inNew Mexico. In the first part of this series,he describes how he planned anddesigned the system.

Photo 1a—Our land is located near Silver City, NM. It’s a perfect spot for living off the grid. b—It didn’t take too long to get started. Here you see the guest house taking shape.

a) b)

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Perhaps it was not impossible.Let’s treat this as a normal design project. What are the

project requirements? We need to design and install an off-grid system that we can afford and that does not changeour lifestyle significantly.

POWER USAGEI first needed to determine if the project was financially

feasible. I started with our current electric usage. Typically,our electric bill is 24 kWh per day. It is more in the sum-mer with air conditioning and also in the winter with morelighting, heating, and indoor living. This is about $120 permonth ($1,440 per year) at current rates. To reconcile thebill with our usage for each appliance, I purchased a P3International Kill A Watt meter on the Internet for about$30 and began to record the power usage for each and everyappliance I could measure. It is a great little meter thatmeasures volts, amps, frequency, watts, and kilowatthours. I left the meter connected to major appliances fora week to get our general pattern of usage. Some appli-ances, such as the electric stove and heating system,could not be measured. The stove is 220 V and the heat-ing system is hard-wired with no place to use the meter.

I next went to the Internet to get typical usages foritems such as the dishwasher, clothes dryer, and oven. Ithen estimated usages for the oil burner, circulatingpumps, and lights. Bottom line: I could account for 22.15of the billed 24 kWh. I feel comfortable that I knowwhere our current usage is spent. This data is in a spread-sheet (EnergyAudit.pdf) posted on the Circuit Cellar FTP

site. How many kilowatt hours are you using? And whereis all that being used?

I thought I had a handle on how our lifestyle consumed elec-trical energy, so I created a column for our new home. Actual-ly two columns. One for phase 1 of the project. We’re buildinga workshop/apartment type home so that we can have a start-ing point for living out west and if we don’t like it, we can getout probably at no financial loss. So, one column is for theworkshop and the other is for the final home. You can see someof the assumptions that we made for each. We need 7.9 kWhfor the workshop and 11.4 kWh for the home. I look at thesenumbers with some reservation because we are not actuallyliving out there and they are estimates. But on the positiveside, appliances are getting more energy efficient each year, so

Lat: 32; long: –108(Solar)Time zone: –8

90˚

80˚

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30˚ 60˚ 90˚ 120˚ 150˚ 180˚ 210˚ 240˚ 270˚ 300˚ 330˚ 360˚

Sol

ar e

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East <--Solar azimuth--> West

12 P.M.

11 AM.

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Jun. 21

May. 21A

pr. 20

Mar. 20

Feb. 20

Jan. 21

Dec. 21

Figure 1—This is a six-month chart of the sun’s path for our latitude and longitude. Note that July to December is the mirror of December to June.

Solar radiation (W/m2)

12/22/07–12/23/07

700

600

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300

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0

10:00 P.M. 2:00 A.M. 6:00 A.M. 10:00 A.M. 2:00 P.M. 6:00 P.M. 10:00 P.M.

Figure 2—Here you see the actual solar radiation recorded by Western New Mexico Universityin Silver City about 30 miles to the west of the house site.

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site for weather (http://weather.wnmu.edu). It displayssolar radiation in watts per square meter. If you look atany of the solar radiation charts on the Internet, you willfind that the southwestern United States is great for pro-ducing solar power. It is even better than islands like

new purchases should be less than what I’ve measured.

ESTIMATING SOLAR RADIATIONThe University of Western New Mexico—which is

located in “Silver,” as the locals call it—has a great web

X-240

PS2AC

#4L1

#2&3N

#1L2

Off50A

On

Off50A

On

Off25A

On

Off25A

On

Off20A

On

Off15A

On

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On

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On

Off15A

On

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On

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Outlet

On

Off50A

Off50A

On

Add NEU/GND bond wire when using PS2AC as load center

Neu

tral

120/240-VACloads

120-VAC X-Fan

120/240-VACAC Source

generator or grid

Byp

ass

AC

In L

EG

1

AC

In L

EG

2

AC

Out

LE

G 1

AC

Out

LE

G 2

Gro

und

Output

Off50A

Off50A

Bypass Input X-240 House loads

AC In

Figure 3—This is an OutBack Power Systems installation diagram of my type of system. Note the current shunt located on the battery’s negative (black) lead. This measurescurrent in and out of the battery. We’ll use this to analyze system operation.

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Hawaii. The University of Oregon has a great sun chartprogram (http://solardat.uoregon.edu/SunChartProgram.html).Figure 1 shows the sun path for our location in New Mexico.

Next, let’s assume that we have no buildings, trees, ormountains to block the sun. This is a good assumptionbecause we are in the middle of nowhere and cloudy daysshould be our only issue. I also used the U.S. solar radiation

resource maps on the Renewable Resource Data Center’s website (http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas)to estimate the solar radiation for our location using a fixedmount tilted at the latitude for the month of December. Theresult was 5 to 6 kWh/m2/day (typical) and 3 to 4 kWh/m2/day(minimum). I figured December with its minimal sunlightshould be the worst-case design. There are plenty of reports

MX60

All GNDs are bounded to the

back plate through the mounting

screwsShown with PV and BAT negatives

internally connected together

B+ PV+B– PV–

B+ PV+B– PV–

PV+ PV+

GNDROD

AC Out

AC In

FX 2024

AC Out

FX 2024

Hub 4 Mate

+

12-V Fan

Batteries

PS2 System

RTS

PS2DC

Off60A

On

Off60A

On

Off60A

On

Off.5A

On

Off60A

On

Off60A

On

Off60A

On

Off

175A

On

Off

175A

On

Copperbus

DC

Pos

itive

OBDC-GFP/2

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on the Internet for solar planning. I’ve just mentioned afew that I found useful.

EQUIPMENTI had great fortune in this area. First, I’m an engineer—and

an electrical one at that (except my degree is so old that itreads “DC ONLY!”). In addition, my neighbors in New Mexi-co already have systems up and running. Jeff and Pat live upthe road in a handmade log home. Jeff is a former engineer forGeneral Motors (Pontiac GTO, Avanti, and Saturn to his cred-it). That makes for some interesting discussions about howelectronics will revolutionize the car industry, but I digress.He’s a mechanical engineer who doesn’t fully embrace all ofthis electrical stuff. He has a minimal system with four panelsof about 150 W each. Another neighbor’s system has about3 kW of panels. Armed with the idea that it could be done,I started to match up equipment with our requirements.

The equipment I selected falls into four main categories:solar panels, inverters, charge controllers, and batteries. Infact, you could consider each independently and not get toofar off an ideal system. There are, however, some areas of con-cern when mating equipment from different manufactures,so I stayed with one manufacturer for the control devices.

SOLAR PANELS Solar panels convert solar energy into electrical energy. Again,

there is a lot of literature available about how this is accom-plished. But what about some hard code conversion details?

Standard test conditions require a temperature of 25°C andan irradiance of 1,000 W/m² with an air mass of 1.5 (AM1.5)spectrum. They correspond to the irradiance and spectrumof sunlight incident on a clear day on a sun-facing 37° tilt-ed surface with the sun at an angle of 41.81° above thehorizon. This condition approximately represents solarnoon near the spring and autumn equinoxes in the conti-nental U.S. with the surface of the cell aimed directly atthe sun. Thus, under such conditions, a solar cell with a12% efficiency and a 100 cm2 (0.01 m2) surface area can beexpected to produce approximately 1.2 W of power.[1]

This gives you an idea of what’s involved in rating andselecting solar panels. Look at the University of WesternNew Mexico’s weather site for solar radiation and you’llget a feeling for the actual solar radiation for the area.

There is another consideration when selecting a panel,namely cost per watt. If you start looking, you will findpanels of different wattages and different prices. In March2004, I started a spreadsheet listing panels from 125 to 195.Note pricing from March 2004, purchased equipment in2005, installed in 2006–2007, and operational in October2007. Then, I added the costs different suppliers werecharging for each panel and calculated a price/watt number.My results range from $4.35 to $4.76 per watt. I estimatedthat I would need 3,000 W of panels, and came up with$13,320 for the cost of the BP Solar SX 170B.

More polysilicon is currently being used in solar panelmanufacturing than all other usages combined, so this is bigbusiness. It also seems that the larger-power-rating panelscommand a higher price per watt. It is sort of like the CPU

business where chips are speed graded and priced accordingly.My cost estimates are a bit old, so you’ll need to run thenumbers with today’s prices. Let me add that I found solarpanels to be in tight supply, so when you begin your design,look to secure the panels at a good price early in the game.

The 3,000 W in my design was derived from the sun’savailability in the winter. Figure 2 represents the solar radi-ation for an actual cloudless winter day. The peak radiationis 600 W/m2. Let’s estimate that the shape of the curve is asine function so that the area under the curve is its averagevalue (2/π, or 0.6366 times the peak value) multiplied byits width. So, that is 600 W/m2 × 0.6366 × 8 h (9 A.M. to5 P.M.), or 3,055.7 Wh/m2. Therefore:

Close to 10 kWh per day is good enough for the workshop,but not enough for the house when it’s built. And 3 kW ofpanels is what one neighbor is using.

We also need to account for cloudy days. The energy torun the workshop would need to come from the battery orbackup generator. Another concern is hot summer dayswhen the panel efficiency drops because of the heat. But thedays are longer in the summer. Actually, it’s still a 24-hourday, but there is more available sunlight each day. I don’thave test results for summer generation (because I’m writ-ing this in February 2008 after getting the system puttogether in October 2007), so stay tuned.

The last point to watch out for in panel selection is cold-weather open-circuit voltage going into the charge con-troller. In the cold, with no current drawn, the open-circuitvoltage of the solar panel will rise. If several panels are con-nected in series (for efficiency), this voltage may damage theinput to the charge controller. This is a well-known situationand your equipment dealer will be able to guide you in this area.

INVERTERSI must confess that I find inverters boring. They are not

as exciting as solar panels, charge controllers, or even bat-teries. I thought I would not find much difference in avail-able inverters and that probably was due to my lack ofenthusiasm. I selected inverters from OutBack Power Sys-tems. I wanted the inverter/charge controller combinationto be from one manufacturer. As I looked at the literature,OutBack seemed to have covered all of the issues for myinstallation. I ended up with two OutBack VFX3648 invert-ers (see Photo 2). They are 3.6 kW (continuous) with con-nections for a 48-V battery and vented. You will find vent-ed and sealed inverters. I selected vented because they typi-cally have a larger power rating and I’m not in a harsh envi-ronment. Also, the inverters are located in an area that isprotected from the elements. Another option is a fan on theinverter. The fan also gives you more capacity, but whatwill you do when the fan fails, and you know it will? Oursystem is a normal 220-V home application. So, there aretwo inverters, one for each phase. OutBack has a neat optionthat includes a transformer to supply the second phase sothe second inverter can remain in a low-power operating

× ×18 panels 170 W/panel

3.0557 kWh/m2 9,350.442 kWh=1,000 W/m2

2807013 martin.qxp 6/10/2008 4:19 PM Page 26

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48.qxp 4/9/2008 9:31 AM Page 1

28 Issue 216 July 2008 CIRCUIT CELLAR® www.circuitcellar.com IEIE

mode. When the power requirements become large enough,the main inverter will signal the second (slave) inverter tostart up and handle the increased load. This is a good setupfor our application. We can install a normal commercialheating/cooling system and power up only the second invert-er when the load is calling for it.

Let me also add here that OutBack has great installationdiagrams on its web site. Figure 3 is the diagram that I usedto configure our system.

The other feature I was looking for in selecting an invert-er was the ability to work seamlessly with a back-up gener-ator. Again, OutBack looked good. The system controllerhas the ability to start the generator when the batteries getlow, warming up the generator under no load, then charg-ing the batteries, and finally letting the generator cooldown under no load before shutting off. Also, the amount ofcurrent drawn from the generator is programmable. Andfinally a generator bypass switch is convenient so we canbypass the system and run the workshop from the generatorin emergencies or during scheduled maintenance periods.

The more I looked into the inverter, the more I becameimpressed with all that was available in such a boringdevice. Inverters just don’t get no respect.

CHARGE CONTROLLEROutBack offers the MX60 charge controller, which is a

real-time maximum power point tracking (MPPT) system.So, as the sun travels across the sky, clouds pass by, solarpanels age, or other real-world events occur, the MX60adjusts the current drawn out of the panels to maximizethe power produced. That sounds good. Now that it’s upand running, it’s neat to watch it work.

The controller’s other significant feature is its efficiency. It’sa buck converter and it will take solar panel output (say 100 Vand 5 A) and convert it to battery input (say 50 V and 10 A) at95% to 99% efficiency. There are many areas to lose powerbecause of inefficiencies in your solar system. This is no longerone of them with the advent of MPPT charge controllers.

BATTERYThe battery is where power is stored for overnight opera-

tion and cloudy days. The best cost/performance battery foroff-grid applications is still the lead-acid type. However, thesebatteries are not your typical car or golf cart battery. Thechemistry is the same but the construction is significantly

different. Each cell is removable and replaceable. In our bat-tery, each cell measures 6″ × 6″ × 21″ and holds 1.7 gallons ofelectrolyte. It certainly isn’t like your car battery; it’s morelike a submarine battery.

The capacity of the battery needs to be sufficient to runthe workshop for three days. And as a matter of efficiency,the larger the battery voltage, the less current flowing in andout of the battery for a given load. We ended up selecting theSO-6-85-17/48 battery from Solar-One. This is a 48-V batterywith 845 Ah (20 h) and 32,448 usable watt-hours (20 h). Itweighs 2,968 lb and has a 9,600-A short-circuit rating. Sun-light might give you a sunburn, perhaps even skin cancer inthe long run, but this battery could kill you in a heartbeat. Icannot stress the importance of following all of the safetyprecautions when you’re around one of these creatures.

The dealer recommended that I get a larger battery, but I heldback based on price alone. (Don’t tell my wife.) The cost of thebattery was $7,000. Time will tell if that was a wise decision.

Let me add another factor I’m still trying to get a handleon: battery efficiency. This is the amount of power you putinto a battery compared to the amount of power you getout of it. Normal lead acid batteries are charged up to the“bulk,” or “absorption,” levels (same meaning just differ-ent names). This is a slight overcharge that is good for bat-tery life but wastes energy. So, I’m not sure how much ofthe energy produced by the panels will be available to bereturned from the battery. This will be measured after thesystem is up and running.

MISCELLANEOUS ITEMSNow all of this has to meet code and be installed in a

cost-effective manner. I decided to use OutBack Power Sys-tems equipment panels for mounting the inverters, chargecontrollers, AC connections and breakers, and DC connec-tions and breakers.

For the solar panels, I went with pole mounts fromPOWER-FAB. There are other items, such as lightningarrestors and relays to start the generator, that I have notmentioned. A good dealer or installer will keep you pointedin the right direction when it’s time to purchase your system.

SYSTEM COSTOK, so how much did all of this cost? That’s a difficult

number to come up with. I can tell you that the equip-ment, including all of the miscellaneous items, costs

Photo 2a—This is where it all began: the placement of the Outback System next to the feed into the normal house distribution panel. b—This was a truly do-it-myself project.Here I’m hoisting up inverters to mount. c—I was very pleased when the AC side was wired. From left to right: AC box, inverters, DC box.

a ) b ) c )

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www.circuitcellar.com CIRCUIT CELLAR® Issue 216 July 2008 29IEIE

George Martin (gmartin@circuitcellar.com) began hiscareer in the aerospace industry in 1969. After five years ata real job, he set out on his own and co-founded a designand manufacturing firm (www.embedded-designer.com).George’s designs typically include servo-motion control,graphical input and output, data acquisition, and remote con-trol systems. George is a charter member of the CiarciaDesign Works Team. He’s currently working on a mobile

PROJECT FILESTo additional files, go to ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2008/216.

$36,000. That does not include the poles for mountingthe solar panels, the backhoe for digging the holes toplant the poles (I ended up with a bigger backhoe thanSteve’s and that gets me in even bigger trouble, as youwill see next time), the trencher for digging the trench tobury the cabling to the panels, the wiring to connect allof the equipment, and the electrician to connect every-thing to code. But seeing the smile on my wife’s facewhen her hair dryer works on a cold winter morning is,well, priceless.

All joking aside, these details can eat into any budget.But I am here early on a winter morning writing this arti-cle, working on the Internet, watching the sun about toshine on the panels and recharge the batteries for anotherday of living and working off the grid.

Next time, I’ll describe the installation process andexplain the system’s performance measurements. So,until then, do your energy analysis and start researchingequipment. I

SOURCESSX 170B Photovoltaic module BP Solar www.bp.com

MX60 Charge controller and VFX3648 invertersOutBack Power Systems, Inc. www.outbackpower.com

Pole mountsPOWER-FABwww.power-fab.com

Kill A Watt meterP3 International Corp.www.p3international.com

SO-6-85-17/48 Battery Solar-Onewww.hupsolarone.com

REFERENCE[1] Wikipedia, “Solar cell,” http://en.wikipedia.org/wiki/Photo

voltaic_cells.

communications system that announces highway information.George is a nationally ranked revolver shooter.

22 Issue 218 September 2008 www.circuitcellar.com IEIE

any local variations. The code is there for a purpose.There may be variation in the details with your localinstaller or inspector, but approaching them in a profes-sional manner will go a long way. Remember, as AlCapone once said, “You can get much further with a kindword and a gun than you can with a kind word alone.”

Last time, I described the planning and design phase ofour off-the-grid solar system in Silver City, NM. Thismonth, I will cover the installation process and explainhow the system operates.

All the mounting, wiring, and enclosures had to com-ply with the latest National Electrical Code (NEC) and

Living and Working Off the GridPart 2: Installation and Performance

INTELLIGENT ENERGY SOLUTIONS

LESSONS FROM THE TRENCHES

by George Martin

George continues describing how heinstalled an off-grid photovoltaic (PV)power-generating system in New Mexico.He details how he mounted the panels,connected the batteries, and monitoredthe system’s performance.

Photo 1—The plywood board is painted white per local code. Here you see the mounting panels and the DC distribution box.

CIRCUIT CELLAR®

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www.circuitcellar.com CIRCUIT CELLAR® Issue 218 September 2008 23IEIE

Refer to the National Fire Protection Association’s website for information on the 2008 National Electrical Code(NEC2008), which is the latest version (www.nfpa.org).

The building where this equip-ment was to be installed was underconstruction during the planningstages of this project, so I was freeto configure things as I saw fit.Actually, one of my original require-ments was that the system wouldn’timpact our lifestyle. As a result, theinterface to the house from the solarequipment panel looks just likepower came in from a utility. I havenormal outlets throughout thehouse. No strange DC outlets or anygoofy low-voltage wiring. I use nor-mal 110/220 all the way.

Let me also add that it soon becameclear that I did not want to try to readand understand the code. So, Iapproached the electrician who wonthe bid on the house’s electrical work.TAZ Electric (Tim Clark and son) usedto be located in Alaska. Installingsolar systems was what Tim did for aliving. Jackpot! He agreed to wire upthe equipment. What a big load off mymind. Also, he was comfortable withme doing as much or as little of thework as I wanted. All of the pieceswere falling into place.

MAIN OUTBACK MOUNTING PANELI purchased the inverters, charge

controllers, AC distribution box, DCdistribution box, and mounting panel from OutBack Sys-tems through The Solar Biz (www.thesolar.biz). The folksthere had good prices, were knowledgeable about solar

installation, and agreed to stage the delivery ofequipment. Plus, it was only about a 2-h drive totheir shop across some beautiful wilderness country.

The interface between the solar AC (output) andthe house AC (input) connects to a normal 200-Acircuit panel that you might find in your home.We should have come right from the OutBack ACpanel and saved this box. Chalk this up to begin-ner’s error. The equipment mounts on two metalpanels. These can hold four inverters. (I’m usingonly two.) I could have gotten away with only oneof those metal panels, but the dealer and electri-cian both agreed that for the price it was better tohave a lot of space to work with.

Photo 1 shows a plywood board (painted whiteper local code) with the mounting panels and theDC distribution box installed. I did this duringone of my cross-country trips. It was actuallypainted in December 2006. The height of thesepanels is also part of the code, so be careful here(41″ off the ground, but check and double checkthis number). You do not want to have to move all

a) b)

Photo 3—Here you can see the battery stand, the engine hoist, the 700-lb. battery, and myable-bodied assistant. Watch those fingers!

c) d)

Photo 2a—This is the starting assembly with the major pieces in place. b—Here you can see more wiring. c—Thisis a close-up of the AC side. d—This is a close-up of the DC side.

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Now both the AC and DC distribution boxes are mount-ed along with both inverters. Time to start the wiring. Theconnections were simple. I suspect that if I did the wiringit would look like a mess, so I called Tim the electrician tosee if he was interested. He was surprised that the equip-ment did not come prewired. It seems that labor is so

of the equipment.

MOUNTING EQUIPMENTI had a platform to mount the

equipment. I started mountingthe inverters in January. I wasthere by myself. Each inverterweighs 62.6 lb! There was noway that I could have lifted thatover my head to bolt it down.Even if I had had help to installit, I wouldn’t have been able toremove them for service. So, Igave up and went back to Con-necticut. We’ve got a bunch ofYankees who hang out at thelocal coffee shop who wouldsurely have the answer. Blockand tackle was the immediatereply from the committee at the coffee shop. Of course, Iwent down to the boat yards and found $100 units. It wastoo much for me, so I went online and found a $20 unitwith a 9:1 mechanical advantage. Refer to Photo 2b in myIssue 216 article to see the block and tackle used to lift oneof the inverters.

Photo 4—Three panels are wired in series and run into the MPPT controllers. Their capacity is 170 W each, 510 W total,to charge the batteries and put off running the generator.

a)

Photo 5a—The poles are in place and one POWER-FAB unit was installed. Oops! b—Here, Steve Becerra and son are lifting out a pole. c—The poles are in their finalplace with a conduit between them. d—These are the trenches from the poles to the house.

b)

c) d)

2809013 martin.qxp 8/11/2008 11:51 AM Page 24

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expensive and hard to find in Alaska that all equipment isprewired and then shipped to the site. He agreed to get all ofthe wire and do it during his next visit to the site. Photos 2a–dshow the wiring internal to the AC and DC boxes.

BACKUP GENERATORI have a generator for the times when we have a week

of clouds or the batteries are so cold they lose their

capacities. To get a partial inspection approval,we connected the generator, bypassed theinverters, and got the inspection for the genera-tor running the house (less batteries and solarpanels). This was a good deal in that the con-tractor and electrician’s work was declaredcomplete and I could get them paid. And it gaveme time to set up the panels. My wife and Iwere only coming out to the site for a couple ofweeks about every three months. So work wasin small increments.

TEMPORARY BATTERY INSTALLATIONThe next step was to temporarily connect the

batteries. Photo 3 shows my wife with one ofthe battery modules, the engine hoist, and astand for the batteries. Each module is a 12-Vbattery weighing 700 lb. The 48-V battery con-sists of four of the battery modules connectedin series. The stand was built to get the enginehoist under the batteries for lifting. I wanted tokeep the batteries out of the way to make solarpanel wiring easier, and then move them totheir final position. In the final position, theyneed to be covered and vented to the outsidebecause of outgassing during charging.

I now had a generator, inverters, and batteries in place. Icould run the generator for 2 h, charge the batteries, andrun the house for two to three days. Water was next. Whilewater is not strictly part of the solar design, I installed a3,000-gallon holding tank approximately 60′ above thehouse. I used gravity to provide water pressure until I couldget the well work completed. So no water pumping isrequired. The well will also be a solar-power unit, but

that’s another article.

SOLAR PANELSThe next and last major items to

install were the solar panels (seePhoto 4). Remember: 18 panels, 170W each, mounted on three poles. Iused pole mounts from POWER-FAB(www.power-fab.com). At about thistime, Steve Ciarcia passed me a copyof his article about his PV system,including information about all of thefuss he made wondering if his poleswere strong enough (S. Ciarica,“Solar-Powering the Circuit Cellar,”Circuit Cellar 209, 210, and 211,2007–08). Thanks for the groundwork, Steve.

I concluded that my panels weresmaller in square surface area, closerto the ground, and they would neversee a hurricane. I hope two out ofthree is sufficient. Actually, the instal-lation instructions were clear on thePhoto 7—This is the PentaMetric system.

Photo 6—This is the maximum power point tracker in operation.

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amount of concrete needed. So, I opened the POWER-FABpackages, assembled what was mounted on each pole, andwas comfortable about the next step, locating and diggingthe holes. With a trusty compass in hand, I located magnet-ic north, corrected for magnetic deviation (14° at my loca-tion), and tested my results against noon shadows. I wasplanning to place the three poles on an East-West line withsufficient spacing (approximately 8″) so that the panelswouldn’t touch or cast shadows on each other.

THE BIGGER THE BACKHOE, THE BIGGER THE MESSI dug the holes with my backhoe, placed the poles, lined

them up, braced them, and called for the concrete pour. Ithen had to wait a day for the concrete to cure. I took thecrew out to dinner. Boy, we were proud of our accomplish-ments. The next day, I set up the panel mounts.

Photo 5a shows the poles with one of them having thePOWER-FAB mount and one solar panel installed. What’swrong with this picture? Notice how the solar panel over-hangs the mounts. I never figured on this overhang. Myspacing was out the window; the poles were too close. Boywas I sad and disappointed. There was only one thing to do:move them. So, we dug new holes at the proper spacingand pulled the pole and concrete base. Photo 5b showsSteve Becerra and his son helping move a pole and base.Well, we did it. We moved two of the poles and bases far-ther apart so that the panels would mount without touch-ing. It took all day, but they are in a straight East-West linewith sufficient spacing.

I placed all of the pole mounts on the poles and mount-ed all of the panels. At the same time, Steve Becerra wasdigging the trenches to connect the pole wiring to thehouse. There are junction boxes on the two outside poles.The 18 panels are connected in six strings of three panelsto a string. Three of the strings run into each of the junc-tion boxes. Photo 5c shows the panels from the polealong with three of the panels from the center pole allwired to the junction box. Photo 5d shows the runs fromthe junction box to the house. I used PVC pipe to encasethe wires. Schedule 40 for above ground and schedule 80for buried pipe.

Tim Clark came out and wired the panels and junctionboxes to the charge controller. Again, neatness and codecompliance was of major concern to us.

CHARGE CONTROLLERSThe solar panels connect to the OutBack MX60 charge

controllers. The charge controllers then go directly to thebatteries. The MX60 MPPT controllers adjust the amountof current received from the panels to maximize the power(volts × amps). Inside is a buck converter switching con-troller to convert solar panel voltage to required batteryvoltage (see Photo 6).

Typical numbers on a bright day in February are 100 Vand 5 A into the MX60 and 50 V and 10 A out to the bat-teries. Charging 48-V batteries, these converters operate at97% or better efficiency.

WIRE SIZINGIt’s an analog world, especially when it comes to wire

size. Small wire is less expensive, but larger wire is moreefficient (less power loss). As you can see in the photos,the battery cables are huge (actually 2/0 or 00 AWG) andonly about 0.2 Ω per 1,000 m. That’s well-suited forcharging the 3,000 W of solar power and 8,000 W of gen-erator power that could be going into the batteries. So,11,000 W and 48 V with two inverters is about 230 Ainto the batteries or about 115 A in each cable.

The other main area of concern is connecting solarpanels to the MX60. Each panel has 12-gauge wire leadsattached. I kept that wire size to the combiner boxes onthe poles. Each solar panel is 35 V and 4.8 A at a maxi-mum power of 170 W. Three panels wired is 105 V and4.8 A. Three of these stacks come into each on the com-biner boxes for a total of 14.4 A to the charge controller.This wire run would be about 70′ to 80′ and consist oftwo wires (power and return). From tables in the MX60manual, for less than 1.5% voltage drop, #8 wire wouldtake you 75′ and #6 would take you 110′.

CONFIGURATION & PERFORMANCEThe inverters came with a predetermined battery volt-

age. I needed to set the charging voltages and chargingtimes, low and high battery voltages, and current limits.The charge controllers first needed to be set up for theproper battery voltage. (All of this is quite detailed andwould change with units from different manufactures. Ifyou are really interested, download the manuals from theOutBack Power Systems web site.)

I added a separate monitoring system. The monitoringsystem is from Bogart Engineering. I selected the Penta-Metric system (see Photo 7). I’ve just begun to learn all ofits capabilities. It is connected to the battery voltage andcurrent shunt measuring current in and out of the batter-ies. The shunt came with the OutBack panel. You can referto it in the system diagram in my July article. The PentaMet-ric also has the computer interface. And Bogart Engineeringsupplies a GUI interface to set up the data that is to be moni-tored and saved. Using the default settings, I started logging

70

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Amps

Volts

Watts/100

Figure 1—This is the output of the solar path program for my location.

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wake up, the television, lights, microwave, hair dryer, andall of the other appliances come on.

Next, the sun comes up, the current swings to over 40 A,and the system is charging. The peak current is middaysun. That current profile is not the sine wave that youmight expect. Once the batteries are charged, the chargecontrollers limit the battery current so they don’t over-charge the batteries. That’s the reduction of current fromthe high to about 5 A positive.

Later, the sun sets and normal evening activities takeplace, which force the current into the negative area, drawstored power from the battery until bed time, and then thecycle repeats.

Look at the graph of the voltage.Over night, it’s about 50 V slowing,going lower as power is pulled fromthe battery. Then, while charging, thebattery voltage increases to close to 60V during the bulk portion of thecharge cycle and then to 56 V duringthe float portion of the charge cycle.And then, as the sun sets and no typeof charging takes place, the battery issupplying all of the power.

The power graph in Figure 1 is cal-culated from the volts and amps. Iscaled it to Watts/100 to fit into thegraph area. All of the power going intothe battery should equal all of thepower coming out of the battery. Asyou look at the data, you might sus-pect that more power is put into thebatteries than is taken out of them. Infact, I added a column to the spread-sheet for cumulative watts (on theCircuit Cellar FTP site). Sure enough,the data supports this suspicion.What’s going on? Well, first, the bat-teries are not 100% efficient. I haveno facts to support this, but I bet youcan look up different battery types andtheir efficiencies on the web. Second,the charging cycle is a controlled over-charge. I’ve read that is to keep thechemistry mixed up and to reduce theformation of sulfates on the plates. Forwhatever the reason, overcharging alsocontributes to some power loss.

I intend to add a second currentshunt to separate the current the solarpanels are providing from the currentthe inverters are delivering. I will alsotune the sampling rates to see if I’mmissing any interesting events.

FINDINGSMy solar system works well. Given the

amount of solar input at my location, I

data. For the most part, this was a boring exercise becausethe days were so sunny and the routine about the housewas so predictable.

The spreadsheet labeled PM_Mar_22_2008_PeriodicData.xlson the Circuit Cellar FTP site contains several days worthof data. The columns titled No Label are unused variablesthat I never did anything with. I was first interested in batteryamps and battery volts. The column titled Watts/100 was cal-culated from amps and volts and scaled to fit in the graph.

Let’s look a bit more into the data. First, notice that theamps line starts out negative at about 4 and continuesuntil it goes negative to about 10. This is the early morn-ing, which means no activity. When the house starts to

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www.circuitcellar.com CIRCUIT CELLAR® Issue 218 September 2008 31IEIE

George Martin (gmm50@att.net) began his career in theaerospace industry in 1969. After five years at a real job,he set out on his own and co-founded a design and manu-facturing firm (www.embedded-designer.com). George’sdesigns typically include servo-motion control, graphicalinput and output, data acquisition, and remote controlsystems. George is a charter member of the CiarciaDesign Works Team. He’s currently working on a mobile

PROJECT FILESTo download the additional file, go to ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2008/218.

SOURCESPentaMetric PM-100D Battery monitorBogart Engineeringwww.bogartengineering.com

SX 170B Photovoltaic module BP Solar www.bp.com

MX60 Charge controller and VFX3648 invertersOutBack Power Systems, Inc. www.outbackpower.com

Pole mounts POWER-FABwww.power-fab.com

SO-6-85-17/48 Battery Solar-Onewww.hupsolarone.com

REFERENCE[1] Wikipedia, “Solar Cell,” http://en.wikipedia.org/wiki/

Photovoltaic_cells.

communications system that announces highway infor-mation. George is a nationally ranked revolver shooter.

could probably reduce the system’s size. But I’m holdingoff until I’ve seen the system operate for a complete 12-month cycle. Remember that being more careful whenselecting your equipment and location can reduce instal-lation costs.

Monitoring solar systems is boring. Nothing is going tohappen overnight, but I suspect you might be able todetect performance reductions such as battery connectionsbeginning to corrode. And this may be a lifesaver.

Batteries take maintenance. I need to add water aboutevery two or three months. I have an automatic wateringsystem that I need to get installed.

I’ve left some solar out of the house equation. I shouldhave solar-heating water to reduce the hot water fuel(propane) and possibly supply some household heating aswell. Look at solar hot water as part of any solar installation.

Yes, it’s just another engineering project, so let’s getgoing. I

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